A self-priming pump is known from DE 10 2007 032 228 A1 and from the subsequently filed WO 2009/007075 A1. With this known pump assembly, the evacuation of the intake side region of the normally-priming centrifugal pump that is necessary for drawing a liquid is realized by the rotating displacement pump upstream from the centrifugal pump. When there is sufficient liquid reservoir in the housing, the rotating displacement pump, which is designed as a so-called liquid ring pump, is able to convey gas, and can accordingly evacuate an upstream process arrangement and draw and convey liquid, or a two-phase stream consisting of a liquid and gas. Once liquid is drawn and enters in the displacement pump and hence the downstream centrifugal pump, flooding the latter, the centrifugal pump basically takes over conveying the liquid or, possibly within limits, the two-phase stream corresponding to its delivery characteristic influenced by the flow loss in the upstream displacement pump.
For continuous availability, the displacement pump always requires the aforementioned sufficient liquid reservoir before an evacuation of the process arrangement connected at the intake side that may become necessary so that the delivery chamber formed by its screw conveyor can ensure the required transport of gas if necessary. In addition to the supply by the intake line of the pump assembly, this liquid reservoir is also fed and maintained by a return line for fluid that on the one hand establishes a connection between a pressure-side interior space of the centrifugal pump (a first connection point, or respectively first connection opening) downstream from the impeller viewed in the direction of flow, and on the other hand the inner space of the housing or the intake port of the displacement pump, or the intake line connected to the latter (a second connecting point, or respectively second connection opening).
Because the liquid reservoir of the positive displacement pump is fed by the return line, it is desirable and advantageous if this return line is primarily supplied with fluid. The delivery of fluid in the return line is, however, necessarily a reflection of the amount of available fluid at the first connecting point, or respectively first connection opening of the return at the pressure-side inner space. Depending on the respective processing conditions, each two-phase stream formed from liquid and gas in given proportions of exclusively liquid up to exclusively gas is delivered by the pump assembly, and hence by the centrifugal pump as well, in the region of the aforementioned pressure-side inner space so that the familiar return line is also necessarily supplied with this respective two-phase stream.
It is known to connect to the return line to the ring channel, which is an integral part of the pressure-side inner space, and to provide the first connection opening in this regard in a lateral boundary surface oriented radially or almost radially, that is part of the rear housing part and borders the face of the ring channel in an axial direction in the form of ring surface. In this ring channel, which can be designed as a spiral ring channel or also as a bladeless ring chamber with a constant passage cross-section, the flow is delayed. This delay converts part of the kinetic energy of the flow leaving the impeller into static pressure so that the overall static pressure rises in the spring channel. A sufficient level of static pressure relative to the static pressure in the displacement pump is needed to transport fluid in the return line. With this arrangement of the first connection opening in the above-described radially, or approximately radially, oriented lateral boundary surface, the fact is exploited that fluid is preferably located in this region, at least with a two-phase stream that is not overly critical, and can be “harvested” there since gas components avoid the rear-most end face wall region of the ring channel, viewed in an axial direction, or bladeless channel if possible.
It is moreover known to position the first connection opening with respect to the pressure port of the centrifugal pump such that an arrangement plane that passes through a radial direction vector that, on the one hand runs through the midpoint of the first connection opening and, on the other hand, through the rotary axis of pump assembly, is penetrated at a right angle by the longitudinal axis of the pressure port. It is preferable because it is recommendable in terms of production to arrange a longitudinal axis of the first connection opening, or respectively the return line, centrally, or nearly centrally, relative to the radial area of extension of the lateral boundary surface.